This invention relates to a push-start type electrically-operated driver which is a manual electrically-operated tool suitable for use in tightening screws and nuts.
Recently, push-start type electrically-operated drivers have been widely used, in which driver an electric motor is rotated upon pushing a bit against a screw and an electric power source is cut off from the motor upon actuation of a clutch when the screw is tightened with a predetermined torque.
An example of a prior push-start type electrically-operated driver will be described hereinbelow with reference to FIG. 9, in which there is generally shown a prior push-start type electrically-operated driver (for details, see Japanese Patent Examined Publication No. 2036/75). In FIG. 9, reference numerals 1, 2 and 3 designate an electric motor, a planetary reduction gear and a clutch, respectively. These elements are covered by a cover 4. Reference numeral 5 designates a switch rod which extends through the planetary reduction gear 2 and motor 1 and is contacted by a switch 7 via a pin 6. The switch 7 serves to rotate and stop the motor 1. Reference numeral 8 designates a bit which is pushed against a screw upon tightening thereof to cause the mechanism of clutch 3 to move the switch rod 5 in the direction shown by arrow a for flipping the switch 7. When the screw experiences a predetermined torque, the clutch 3 causes the switch rod 5 to be moved in the direction of arrow b, so that the switch 7 is turned off to stop the electric motor 1.
With the prior arrangement described above, however, the shaft of the motor 1 must be hollow so as to pass the switch rod 5 therethrough, which is quite disadvantageous in terms of the manufacturing cost and rigidity in the case of electric motors having a large length. In addition, the longer the switch rod 5 the larger the curvature thereof becomes, so that its sliding movement through the motor shaft and the planetary reduction gear is not smooth to make switching unstable. Furthermore, the state of things in the prior art is that even if the switch rod 5 extending through the motor 1, planetary reduction gear 2 and clutch 3 is good in precision with respect to its longitudinal dimension, the switch 7 is excessively or insufficiently pushed by the rod 5 due to the accumulative errors produced by tolerances of parts dimensions of the three elements, that is, the motor 1, planetary reduction gear 2 and clutch 3.